What result i should return?

Question

I write simple library wich returns List of names.

But,what i should returns if i can not find anything?

return new List<String>();

or

return null;

example:

var resultColl=FindNames(...);

This code can be used from another components and i do not want to corrupted it. If i return null- i think it is right way to check it. But, may be i should return empty list?

Thank you.

Note, that you shouldn't return `List` at all. Return empty `IList` or, if it is possible, empty `IEnumerable` instead. — Dennis, Sep 25 '15 at 12:21

score 15 · Accepted Answer · answered Sep 25 '15 at 12:16

15

You should always return empty list. See the Guidelines for Collections.

DO NOT return null values from collection properties or from methods returning collections. Return an empty collection or an empty array instead.

answered Sep 25 '15 at 12:16

Rahul Tripathi

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absolutely, avoid using `null` as much as possible. it's just library should offered some default empty implementations. – zinking Sep 30 '15 at 05:15

npinti · Answer 2 · 2015-09-25T12:39:57.940

5

I'd return Enumerable.Empty<string>(), or if your method is supposed to return a List, do return new List<string>(). If you have a lot of situations where you need to return an empty List, you could create a static list which is returned each time, this would stop new empty lists from having to be created each time, as pointed out by @YuvalItzchakov.

An empty collection is better than null since it would, in my opinion lead to cleaner code.

edited Sep 25 '15 at 12:39

answered Sep 25 '15 at 12:16

npinti

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You probably ment `Enumerable.Empty().ToList();` – Yuval Itzchakov Sep 25 '15 at 12:31
@YuvalItzchakov Why? If you go that route, just do `new List();` – Rob Sep 25 '15 at 12:31
1

@Rob Unless the OP is receiving an `IEnumerable`, this wont compile. As he used `List`, i assumed that's what he's using. – Yuval Itzchakov Sep 25 '15 at 12:32
@YuvalItzchakov: I do not know what is the OP expecting, but if `List` is expected then it would need to be converted, but then again it would make the use of Enumerable.Empty redundant. – npinti Sep 25 '15 at 12:36
@npinti In that case, I think creating a `static List` would be better, performance wise. – Yuval Itzchakov Sep 25 '15 at 12:37
@YuvalItzchakov: Yes it would. – npinti Sep 25 '15 at 12:39
Unless the caller decides to modify the returned list afterwards - then returning static List would be a disaster. – May 30 '18 at 16:41

score 5 · Answer 3 · answered Sep 25 '15 at 12:29

Returning the empty list is more convenient for users of your function:

foreach (string name in FindNames(...))
{
   Display(name);
}

Returning null forces your callers to write extra code:

a test for null, and

an extra local variable (to avoid having to call your function twice)

List<string> names = FindNames(...);

if (names != null)
{
   foreach (string name in names)
   {
      Display(name);
   }
}

So returning an empty list is better.

score 3 · Answer 4 · answered Sep 25 '15 at 12:17

It's likely that the calling code will want to iterate the list, or do something with a List. By passing back an empty List the calling code should work fine. If you return null then the calling code will have to ensure they have a list first.

It's probably just a matter of preference, but returning an empty list gets my vote ... you're returning the thing the contract said it would return.

score 2 · Answer 5 · answered Sep 25 '15 at 12:15

2

Returning empty collection is better from the design perspective because client code doesn't need to perform null checking. See Null Object pattern.

answered Sep 25 '15 at 12:15

Krzysztof Branicki

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score 1 · Answer 6 · answered Sep 25 '15 at 12:18

1

That depends on the semantic of your code.

If no results is an acceptable result, then you should return an empty collection.

If no results is an error condition, then return null.

answered Sep 25 '15 at 12:18

Domysee

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If no results is an error condition, consider throwing an exception! – Matthew Watson Sep 25 '15 at 12:22
@MatthewWatson you're right, exception would be better than returning null – Domysee Sep 25 '15 at 12:30

score 1 · Answer 7 · answered Sep 25 '15 at 12:21

I would suggest that you be consistent in what you do when you have nothing to return. This way you always know for anything that you are calling to expect the same type of response for nothing. Such as knowing that you will always return an empty set or an empty string or 0 etc.

What do you do in regards to the other library items you have?

tomab · Answer 8 · 2015-09-25T12:32:30.363

0

A more detailed answer would be a Tuple<bool, List<string>>. This is a clear solution which can be modified to include other details about your search:

var thereAreResults = foundList.Count > 0;
return new Tuple<bool, List<String>>(thereAreResults, foundList);

edited Sep 25 '15 at 12:32

answered Sep 25 '15 at 12:16

tomab

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No reason from the downvoter? – tomab Sep 25 '15 at 12:23
1

That's a horrible horrible design. It also doesn't compile. – Rob Sep 25 '15 at 12:30
1

I didn't down vote, but this seems pointless since the caller can just call `Count` on the returned list or more likely iterate it in a `foreach`. Also you could just `return Tuple.Create(thereAreReullts, foundList);` instead. – juharr Sep 25 '15 at 12:30
The emphasize was a way to include more information than just an empty List(). But nevermind. – tomab Sep 25 '15 at 12:38

What result i should return?

8 Answers8